Neutron beam, thermal

SOURCE OF COLLIMATED THERMAL NEUTRON BEAM BASED ON A NEUTRON GENERATOR FOR NONDESTRUCTIVE EVALUATION OF MATERIALS AND PRODUCTS... 

NAA has been most frequently associated with the nuclear reactor as a tool because of the availability of intense beams of neutrons at thermal energies (0.025ev) from such facilities. Many elements have a reasonably large probability (cross-section) for thermal neutron capture. 

Neutron depth profiling technique (NDP) [13]. NDP is a speeial method for depth profiling of few light elements, namely He, Li, B and N in any solid material. The method makes use of speeifie nuelear reaetions of these elements with thermal neutrons. The samples are plaeed in the neutron beam from nuclear reactor and the charged products of the neutron indueed reactions (protons or alpha particles) are registered using a standard multiehannel spectrometer. From the measured energy spectra the depth profiles of above mentioned elements can be deduced by a simple computational procedure. 

BNCT using thermal neutron beams was started in the United States in 1951 at the Massachusetts Institute of Technology (MIT) and at the Brookhaven National Laboratory (BNL). The clinical results were very poor. The technique was introduced in Japan by Hatanaka in 1968, and some promising results were obtained. 

BNCT was restarted in the United States in September 1994 at Brookhaven National Laboratory and shortly thereafter at MIT using epithermal neutron beams (BNL trials ended in 1999 after the treatment of 53 patients but continued at MIT) these programs are supported by the Department of Energy. Forty patients were treated by the end of 1997. In Europe, the European Commission supports a BNCT program in Petten, The Netherlands. The three first patients were treated in 1997. The thermal neutron beam program continues in Japan. 

Lastly, BNCT is used today in combination with fast neutron therapy in some centers such as Seattle, Essen, and Orleans. Boron is incorporated in the tumor cells it captures thermal neutrons produced in the body by the fast neutron beam. Combination of BNCT and external photon beam therapy has also been suggested [51]. 

Figure 5. The measured attenuation coefficient for die NIST MCP detector at die BT2 thermal neutron beam and the NG1 cold neutron beam.

Our studies have involved the use of both neutron and charged-particle beams and Table I presents a partial list of the targets prepared. The Nobel gas targets were prepared for in-beam y ray studies which used an external thermal neutron beam. Large amounts of target material were required for these experiments and a special device - a cryostat - was constructed to isolate and contain a large quantity of gas (in the solid state) so that the measurements could be done. The details of the construction of this device,... 

Inelastic neutron scattering, on the other hand, usually employs a monochromatic neutron beam and records the intensity of the scattered neutron beam as a function of neutron kinetic energy. Such inelastic collision spectra are monitored as a function of the applied field and the (usually low) temperature. The observed peaks then represent the energy differences of thermally populated and excited unpopulated multiplet states. Inelastic neutron scattering experiments can be conducted using triple-axis, backscattering, or time-of-flight spectrometers. 

The base metal and the welded joint have been thermally treated, cooling down in water from the initial temperature 1050°C. The welded joint has been scanned by a thin neutron beam to obtain the SANS data for base and welded metal. The cross sections showed three fractions of particles with gyration radii Rgi 20 nm, Rg2 9 nm and Rg3< 1 nm (point like defects). The amount of these defects in base metal was larger by factor 5, as compared to welded metal (see Fig. 8). 

On a molecular scale liquid surfaces are not flat, but subject to Jluctuations. These irregularities have a stochastic nature, meaning that no external force is needed to create them, that they cannot be used to perform work and are devoid of order. Their properties can only be described by statistical means as explained in sec. 1.3.7. Surface fluctuations are also known as thermal ripples, or thermal waves, in distinction to mechanically created waves that will be discussed in detail in sec. 3.6. Except near the critical point, the amplitudes of these fluctuations are small, in the order of 1 nm, but they can, in principle, be measured by the scattering of optical light. X-ray and neutron beams. From the scattered intensity the root mean square amplitude can be derived and this quantity can, in turn, be related to the surface tension because this tension opposes the fluctuations ). 

At a spallation source a heavy-metal target, such as Pb, W, Ta or Hg, is bombarded with energetic particles, usually protons accelerated to energies of up to 1 GeV. Neutrons freshly released from an atomic nucleus have high energies, referred to as epithermal neutrons , and must be slowed down to be useful for powder diffraction experiments. This occurs by collisions between the neutrons and the moderator - such as liquid methane or water - placed in the path of the neutron beam, which cause the exchange of energy and a trend towards (partial) thermal equilibrium. 

Neutrons in thermal equilibrium at 298 K can be used for diffraction in a similar way to X-rays, since they also have wavelengths comparable to interatomic spacings. In contrast to X-ray diffraction, the powder neutron diffraction experiment is much more common than single crystal neutron diffraction, since the beam intensity tends to be 1000 times less than for X-ray diffraction, so that single crystals of a sufficient size to collect good data are difficult to grow. 

A schematic view of a generalized neutron experiment is shown in Figure 1, along with the essential quantities defining the incident and scattered neutrons. A collimated white thermal neutron beam from a reactor is impinged on a single crystal monochromator or a mechanical... 

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